The present invention relates to a making of shaped articles from a mixture composed of fibrous material and a thermosetting binder. More particularly, the invention relates to a method of making such shaped articles.
The making of shaped articles having a profiled configuration from mixtures of this type is already known in the art. Conventionally (and it should be noted that the following mixture compositions can also be used in the present invention) various types of fibrous materials such as lignocellulose fibrous materials including wood chips, sugar-cane fibers or the like, are mixed with a thermosettable synthetic plastic resin, such as melamine, ureaformaldehide or phenolformaldehide. It is also known to use other types of fibrous materials, such as glass fibers, rock wool or asbestos fibers, or to use any of these fibers in various combinations with one another and in admixture with a thermosetting binder.
The prior art teaches to fill the mixture into a preliminary mold to a thickness corresponding to approximately 6-10 times the thickness desired for the finished profiled or shaped body. Thereupon, the mixture is compressed in the preliminary mold in cool condition and to such an extent that it forms a blank having almost the shape and dimensions of the desired shaped article. This blank is then removed from the preliminary mold and, since it has only been cold pressed and the thermosetting binder has not hardened, the blank tends to expand as soon as it is removed from the preliminary mold, but only to a relatively slight extent; this is known as swelling or breathing of the blank. The blank swells to such an extent that its exterior dimensions are about 10-20% larger than the dimensions which are required for the finished shaped article. The blank is thereupon inserted into an appropriately shaped and dimensioned cavity of a hot-pressing mold and is again compressed under application of heat, until it assumes the shape and dimensions required for the finished article, and during this hot-pressing operation the thermosetting binder sets and hardens. During the hot-pressing operation, the shaped body may also be provided with a decorative cover layer that is pressed onto its surface, if desired.
This method of making shaped bodies from mixtures of the type in question, and the equipment for carrying out the method, is very widely used for the manufacture of large bodies, such as tabletops, wall mouldings and the like. In the manufacture of these articles, it is of little or no economic importance that the cold pressing and the subsequent hot pressing are carried out in separate and entirely independent steps, and that in between these steps the cold-pressed blanks swell to some extent for the reasons and in the manner described earlier.
However, when relatively small shaped bodies are produced, for instance decorative grilles, circular members or the like, these two factors become of very considerable importance. The prices at which such relatively small articles can be sold are such that the labor-intensive prior method requiring two separate and distinct pressing steps, frequently makes the operation economically impractical. A different disadvantage, or sometimes an additional one, may also be the fact that the swelling that takes place in the blanks after the cold pressing and before the hot pressing step, is often objectionable in small shaped articles since they can no longer be precisely inserted into the mold cavity or cavities of the hot-pressing mold, so that the article obtained by completing of the hot-presing operation will be of inferior quality and may have flashings due to improper fit in the cavity of the hot-pressing mold.